Drugs are frequently eliminated from the body by two or more organs, the relative contributions of which depends on the route of administration of the drugs. A study is in progress of the relationships between steady-state blood concentrations and urinary and biliary excretion of p-aminohippurate (PAH) during simultaneous infusion into the portal vein of PAH labeled with 3H or 14C and into the tail vein of PAH labeled with the other isotope. This study serves as a surrogate for a drug that is metabolized by the liver and excreted by the kidney.